Often it is necessary to provide a visual warning indication to indicate either the presence of a particular hazard, gain a person's attention or warn someone away from a given area. In many instances, a large intense light source such as a spotlight or other warning device may be used that is attached to a support structure and powered from a separate power supply. However, in many instances there is the requirement for a handheld or manually operable device having similar dimensions to a torch or the like which can be carried or alternatively attached to another manually operable piece of equipment.
One example of such a requirement would involve a security guard monitoring a checkpoint to prevent the entry of unauthorised vehicles or persons to a location. In this scenario, a security guard, who will typically be on foot, will often need to provide a visual warning indication to alert the driver of a vehicle or other persons approaching the checkpoint to stop or otherwise slow down. While commonly a standard handheld torch is used, these devices often cannot be seen, especially in bright ambient conditions or in those environmental conditions where there may be dust due to weather conditions or background smoke or haze. In addition, where vehicles may be moving at speed then the visual warning indication needs to be visible from distances of at least two to three hundred meters to function effectively. To address this deficiency of standard hand held illumination devices, a number of laser based warning devices have been developed.
Laser based devices however, suffer from a number of serious disadvantages. While these devices are clearly able to provide an intense visual indication, they typically share the common problem that within a certain distance from the device (e.g. up to 100 meters) they are no longer eye-safe. This means that a person exposed to the visual warning indication emitted by the device is likely to suffer from eye damage. Not only does this present a hazard to the person being warned but in addition the operator of the warning device must take extensive precautions not to accidentally illuminate themselves or anyone else within the hazard range of the device. This problem of laser based devices is in large part due to their point source nature resulting in the power of the visual warning indication being tightly focused on the retina, thereby resulting in damage to the ocular system.
In one attempt to address this problem of current laser based systems, the laser intensity is simply reduced to an eye-safe level by reducing the power of the laser employed. However, this results in a reduced effective range for the warning device. Another attempt to make these devices eye-safe involves reducing the fraction of total laser power which can be can be captured by the fully opened iris of the eye in normal operation conditions. This is achieved by expanding the effective size of the laser beam at the output aperture of the device. However, this involves the introduction of bulky optical elements which increases the overall weight and bulkiness of the visual warning device, thereby detracting from the ease of use of the device.
Other more complicated designs have attempted to use multiple point laser sources each of lesser intensity resulting in complicated optical arrangements that add extra cost and size to the device. In yet another attempt to produce a compact eye-safe visual warning device, some devices employ an extended laser source by degrading the laser beam quality by the insertion of an optical element which functions to blur the focal spot. However, this once again increases the size of the output optics. There is accordingly a need for an optical device for providing a visual warning indication that is relatively compact and that may be used in a range of circumstances.